Multi-stroke engine operation (e.g., varying the number of strokes in a combustion cycle) is one method that may improve fuel economy while maintaining reserve torque capacity for elevated load conditions. During Multi-stroke operation, selected strokes of each cylinder may not follow a typical four-stroke cycle for some conditions but rather perform addition compression or induction strokes, for example. Operating an engine in this fashion can extend the torque interval, thereby lowering the average torque available from an engine. However, it can also increase fuel economy, at part load, by increasing the cylinder air amount and thermal efficiency. Further, the cylinder may resume four-stroke operation by simply eliminating any benign pumping strokes after a combustion event in the multi-stroke cylinder. Consequently, the ability to control the stroke count, e.g., 2-stroke, 4-stroke, 6-stroke, and/or 12-stroke, can enable cylinders on-demand to be used to improve fuel economy while retaining torque capacity.
One method to control intake and exhaust valve operation during engine operation is described in U.S. Pat. No. 5,743,221. This method provides a method of controlling valves in a cylinder as a function of desired engine load requirements. The method controls cylinder torque by up to three control processes, full open and full close valve operation, partially open and full close valve operation, and open or closed valves with deactivated fuel. The method may be employed in the case of four-stroke or multi-stroke cylinder cycles.
However, the inventors herein have recognized that the before-mentioned approach can have several disadvantages. Namely, the valve control process is determined simply based on desired engine load requirements. Consequently, the method may provide less than desired torque if a valve in the engine has experienced degradation. If a degraded valve is operated by the method, the degraded valve may not provide enough air to produce the desired torque. Furthermore, a cylinder inducting less air than desired may combust a rich air-fuel mixture that may lead to increased emissions.
In addition, the method adjusts cylinder torque by using up to three valve control processes, but electromechanical valves may provide even finer torque resolution than that described by the before-mentioned approach.